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UC researchers announce results that could complicate
measures to halt spread of Sudden Oak Death 10 Jan 2001 By
Catherine Zandonella, Media Relations Berkeley - A common
nursery plant may lead to increased complications and
possible new management practices in the fight to halt
Sudden Oak Death, a highly contagious fungal disease that is
killing California oak trees, University of California
researchers announced today (Wednesday, Jan.10).

In a breakthrough in the study of the disease, UC
researchers discovered that the rhododendron, a popular
ornamental plant, can be infected by the same fungus that is
causing the oak disease.
The fungus has infected European rhododendrons and, as of
yesterday, the researchers confirmed that it also is
affecting California rhododendrons, suggesting a
transcontinental link.  Finding this relatively new fungus
in two different parts of the world - and in two species -
is unusual, the researchers said.

The rhododendron discovery gives insight to the potential
origin and transmission of this pathogen and may suggest new
ways of spread.
Previously, the pathogen only was known in three other
California oaks -
tanoaks, coast live oaks and black oaks.

"We now know we have a host that could have carried the
fungus a long way," said Matteo Garbelotto, a plant
pathologist and adjunct professor in the Department of
Environmental Science, Policy & Management in UC Berkeley's
College of Natural Resources.
"People don't really export oak trees across state lines or
around the world,"
he said, "but they export rhododendrons."

The finding may have a major impact on how scientists manage
the disease.  Co-investigator David Rizzo, assistant
professor of plant pathology at UC Davis, said it may result
in new restrictions on the rhododendron nursery industry.
"The big concern is that someone will transport a sick
rhododendron to a place where there are susceptible oak
species," he said.

The breakthrough came when a Clive Brasier, British
researcher who had visited UC Berkeley last summer, later
noticed in Europe a fungus that looked like one he'd seen in
Garbelotto's lab.
The European fungus had been found on rhododendrons in
Germany and the Netherlands.  Brasier contacted the UC
scientists, and researchers from all four countries
determined together that the European rhododendron fungus
was identical to the California oak-killing agent.  This
finding established that the fungus is not exclusively found
in California and has important implications for
international trade.

But Rizzo and Garbelotto needed more proof to confirm the
link between the two plant species, and yesterday they got
it.  Rizzo and Steve Tjosvold, a Santa Cruz County farm
advisor, found the fungus in a rhododendron taken from a
Santa Cruz County nursery, and Garbelotto confirmed with DNA
analysis that it was the same fungus killing the oaks.

The scientists don't know whether the disease was
transmitted from California to Europe, or vice versa, or
whether it traveled to both places from a third, as yet
unknown, location.  The fungus, first noted in European
rhododendrons in 1993, has not been found in European oaks.
However, European scientists are concerned that the disease
will spread to European oak forests, particularly those in
areas with a climate similar to that of California.

Since the discovery of the mysterious oak-killing illness in
California in
1995, researchers have been scrambling to understand the
disease and design strategies to stop its spread.  It is not
known if the fungus recently was introduced into California,
or if it is a native fungus that recently became a
tree-killer because of environmental changes.  Tens of
thousands of oak trees have succumbed to the disease, and
the researchers have reported up to 80 percent mortality in
some infected groves.

Through molecular sleuthing, Rizzo and Garbelotto determined
that the disease was caused by a never-before-seen strain of
fungi from the genus Phytophthora (Phy-TOFF-thoruh).  A
relative belonging to this 60-member group caused the Irish
potato famine, and another relative is linked to the dieback
of cedar trees in Northern California and southern Oregon,
eucalyptus trees in Australia and oaks in Mexico, Spain and

In California, Sudden Oak Death has been reported from
Sonoma Valley in the north to Big Sur in the south, a
190-mile range, as well as east to the Napa County border,
about 25 miles inland.  The hardest hit counties are Marin
and Santa Cruz.  The disease affects tanoak (Lithocarpus
densiflorus), coast live oak (Quercus agrifolia), and
California black oak (Quercus kelloggii) found along the
coastal belt in California.  To date, the disease has not
been found in other oaks such as blue oak or interior live

The dieback is alarming, researchers say, for its potential
to disrupt the coastal forest ecosystems.  Oaks provide
habitat for wildlife and a food supply for small mammals and
are frequently planted as ornamentals in gardens and parks.
Additionally, downed dead trees create a fire hazard from
the resulting buildup of dry fuel.

There are similarities between the disease in oaks in
California and rhododendron in Europe.  In both cases, the
fungus attacks above ground parts of the plants.  In oaks,
the fungus enters through the trunk and causes the formation
of bleeding cankers on the trunk.  On rhododendron plants,
the fungus causes similar cankers and spreads from twig tips
to the stem base, according to the European researchers.

The researchers have notified agricultural and ecosystem
managers in the affected areas of the rhododendron
Research is underway to determine if native rhododendrons -
those that have not been imported -
are being infected.  Research also is being conducted to
determine how many other susceptible species may be affected
by the fungus.
Dr.  Andrew J.  Storer Division of Insect Biology
201 Wellman Hall University of California Berkeley, CA
94720-3112 USA Tel: (510) 642 5806 Fax: (510) 642 7428 email


There is further information regarding topic at


From what I understand from the information available is
that it appears to be almost totally destroying the 3
species of oak named in the report that are native to
California.  So far, European oaks have not contracted it,
but rhododendrons in both California and Europe have been
infected with it.  It is a relatively new fungus.  It would
appear that the rhododendron is the host plant, given that
oak trees aren't exported.

The same (exact same DNA) Phytophthora has been known in
European rhododendrons since 1993 but is not the same as
other Phytophthora strains that also affect rhododendrons.

The mystery is, where did it originate? ..... if it didn't
come from Europe via rhododendrons shipped to California,
did it travel to Europe from California?  Or is there a
third locale, such as rhododendron collections from Asia?
Perhaps there is another host species which is contaminating
both rhododendrons and oaks.   Perhaps it is a native fungus
that went through a mutation due to environmental changes or
proximity of new host species.

To answer your question, it does not seem to be a soil-borne
fungus as it attacks above-ground parts of both plants:  in
oaks, through the trunk causing bleeding cankers on the
trunk;  in rhododendrons it causes similar cankers and
spreads from twig tips to the stem base.

There is more information at other sites  - will find the
links again.
Diana Pertson
Jan 2002
I have been following this since it was first reported and 
communicated with Dr. Storer to get info on its relation to 
rhododendrons so I could alert members of the ARS.  In my 
opinion, the pathogen is a new discovery but may not be a 
new disease.  There are many other diseases of oaks and 
rhododendrons.  Oaks in particular have been suffering from 
these for a long time.

Diane Pertson

The following is from 'more background information' at:

Clive Brasier ( Forest Research, Alice Holt Lodge, UK) has
determined that a new species of Phytophthora isolated as
early as 1993 from ornamental Rhododendrons in Germany and
The Netherlands matches the newly-discovered species found
in California.

This new Phytophthora appears to be genetically distant to
most of the other 60 species within the genus Phytophthora.
The closest relative in the genus is Phytophthora lateralis,
a virulent pathogen of Port Orford Cedar known to be present
in natural stands of the Pacific North West and occasionally
on Port Orford Cedar stock in nurseries in Europe.

          The presence of the same pathogen in two discrete
and extremely distant areas raises important issues on the
potential mode of transfer of an aggressive pathogen between
continents but provides no information on where the pathogen
may have originated.  The pathogen may have originated in
either known areas of distribution or in a third region of
the world.

          In  California, there is a substantial overlap
between areas in which rhododendrons grow and areas in which
the oak species known to be susceptible to the new disease
are found.  Rhododendrons are also extremely common
ornamental species throughout the state.

The California report mentions that the spread may be from spores 
that are splashed by raindrops from the soil rather than airborne. 
In any case any source of spores, soil or diseased plants, would lead 
to spread.

"The pathologists on the team have isolated an important causal agent 
- Phytophthora, and beetles, other fungi, and weather may be 
additional factors. Phytophthora is a fungus that appears to enter 
through the bark on tree trunks and limbs, possibly after they are 
splashed there by raindrops. Once in the tree, the fungus produces 
enzymes that dissolve the dead outer and living inner layers of bark. 
Oozing sores result as the cell walls break down. (from the UC Davis 
Press Release).

Once the trees have gone through the progressive stages of the 
symptoms, their vigor rapidly declines and they become vulnerable to 
secondary insect pests such as bark and ambrosia beetles. 
Pseudopityophthorus (bark beetle) occupies the phloem-xylem portion 
of a tree, at the bark-sapwood interface, whereas ambrosia beetles 
bore deep into the heartwood of the tree, and kill it by blocking its 
circulatory system. These beetles are not known to be capable of 
invading healthy trees but can be very destructive if trees are 
weakened. As the number of dead and beetles-infested trees increases, 
this may provide a reservoir of colonizing beetles that can attack 
other weakened trees."

Cheers, Steve Henning in Reading, PA   USA


There was a previously unidentified phytophthora infection in
rhododendrons in Germany and the Netherlands, first reported in 1993 .
A botanist researcher at the University of California at Berkeley,
noticed in late 2000 that it closely resembled  the then unidentified
fungus in California oaks.  DNA tests then revealed that it was the same
species of phytophthora.   Any suggestion that it spread from California
to Germany or vice versa is pure speculation at this point.  It could
have been around in another host plant for eons, and then moved into
rhododendrons oaks, and later many other California species.  DNA-wise,
it is very closely related to Phytophthora lateralis which infects the
Port Orford cedar in the US Pacific northwest.  It is possible it
mutated from that species.

Its effects on rhododendrons has been observed here in California.  It
is not lethal to the plant, and may be treated (in rhododendrons) by
application of fungicides (I don't remember which ones).
A website with links to further information:

Richard Starkeson